Two experimental series of short-span IBWB specimens fabricated from 3-mm-thick steel plates were tested to evaluate their behavior under combined bending and torsion. Series 1 comprised hollow IBWB sections, while Series 2 included identical specimens with box webs filled with normal-strength concrete to investigate composite action. The width-to-depth (W/D) ratio varied from 0.70 to 1.30 while maintaining a constant steel area, ensuring that geometric effects could be isolated. All specimens were subjected to identical loading conditions. For the hollow beams, increasing the W/D ratio significantly enhanced torsional performance. Specimens with W/D ratios of 1.30 and 1.15 achieved increases in torsional capacity of 76.27% and 26.32%, respectively, relative to the control beam. Conversely, specimens with lower W/D ratios (0.85 and 0.70) showed only minor strength improvements or slight reductions and consistently exhibited lower rotation capacities, indicating reduced ductility. The concrete-filled specimens demonstrated the beneficial effects of composite action. The beam with W/D = 1.30 achieved a 25.91% increase in torsional strength and a rotation angle exceedingly twice that of its hollow counterpart, indicating substantial improvements in both strength and ductility. Although some concrete-filled specimens experienced slight reductions in torsional strength, they generally displayed enhanced deformation capacity. Compared with equivalent hollow sections, all concrete-filled beams exhibited significant increases in torsional capacity, ranging from 182% to 213%. Overall, geometric configuration strongly influenced torsional behavior. Higher W/D ratios improved strength and stiffness, while lower ratios provided limited benefits. Concrete infill markedly enhanced torsional resistance, delayed local buckling, and improved overall structural performance, particularly in specimens with larger W/D ratios.
